I'm not a lawyer either, this is not legal advice, and also it's a bit off the thread originator's topic. That said: in the U.S. the 1923 rule is probably fairly strong for things like books and stories that were published by reputable U.S. publishers. But of course the copyright is expired only in the book as it was published prior to 1923. If a "revised edition" was published in 1923 or later, the revisions have a separate copyright applying only to them. The problem, of course, is deconvoluting them from the uncopyrighted parts.
Of course the "1923 rule" (or "1922 rule" as I prefer to name it, after the last year from which works were promoted to the public domain) applies only to the U.S. In other countries a copyright might last longer, or not as long. Also the copyright status of some works that were first published outside the U.S. is complicated in the western U.S. by a bizarre court decision (Twin Books--the Bambi case) in the U.S. ninth judicial circuit. Then there are at least hypothetical complications involving the old common-law right of first publication. Still (though I repeat that I am not a lawyer, this is not legal advice, etc. etc.) the "1922 rule" strikes me as being a part of copyright law that is somewhat less unpredictable than, say, the answers to questions of fair use.